Etchant for use in a semiconductor processing method and system

ABSTRACT

A method and system for processing a substrate in the presence of high purity C 5 F 8 . When processing oxides and dielectrics in a gas plasma processing system, C 5 F 8  is used in combination with a carrier gas (e.g., Ar) and one or more of CO and O 2 . When using a silicon nitride (Si x N y ) layer as an etch stop, effective etching is performed due to the selectivity of oxides versus silicon nitride. The method is used when etching down to self-aligning contacts and other layers. The method may be practiced with or without using an anti-reflective coating underneath the photoresist layer.

[0001] The present application claims priority under 35 U.S.C. 119 toJapanese patent application 9-368081 filed Dec. 27, 1997, the contentsof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention provides a semiconductor plasma processingmethod and system using novel combinations of gases including C₅F₈. Inan exemplary preferred form of the invention, C₅F₈ is utilized incombination with CO and/or O₂ in the presence of a carrier gas, e.g.,Argon.

[0004] 2. Discussion of the Background

[0005] Integrated circuits and other electrical devices are todaymanufactured utilizing plural plasma processing steps, in which theplasma interacts with a substrate (e.g., a semiconductor wafer) to (1)deposit material onto the substrate in layers, or (2) etch the variouslayers formed on the substrate. Deposition and etching are not alwaysmutually exclusive since, e.g., during an etching operation, materialscan also be deposited.

[0006] When fabricating semiconductor devices, numerous regions havingdifferent electrical properties (e.g., conductive regions,non-conductive regions, etc.) are formed in layers on and upon thesemiconductor substrate. The conductive regions include a semiconductorsubstrate, a source or drain region, the gate material of a gateelectrode, and a conductive material. Non-limiting examples of suitableconductive regions include a metal such as aluminum, polysilicon (whichmay be conventionally doped with n-dopants such a phosphorous, arsenic,antimony, sulfur, etc., or with p-dopants such as boron), titanium,tungsten, copper, and conductive alloys thereof such as aluminum-copperalloy and titanium-tungsten alloy.

[0007] The conductive regions and layers of the device are isolated fromone another by a dielectric, for example, silicon dioxide. The silicondioxide may be (1) grown, (2) deposited by physical deposition (e.g.,sputtering), or (3) deposited by chemical deposition. Additionally, thesilicon dioxide may be undoped or doped, for example, with boron,phosphorus, or both, to form borosilicate glass (BSG), phosphosilicateglass (PSG), and borophosphosilicate glass (BPSG), respectively. Themethod used to form and dope a silicon dioxide layer will depend uponvarious device and processing considerations. Herein, all such silicondioxide layers are referred to generally as “oxide” layers.

[0008] At several stages during fabrication, it is necessary to makeopenings in a dielectric layer to contact underlying regions or layers.Generally, an opening through a dielectric layer between polysilicon andthe first metal layer is called a “contact opening,” while an opening inother oxide layers such as an opening through an intermetal dielectriclayer (ILD) is referred to as a “via. ” As used herein, an “opening”will be understood to refer to any type of opening through any type ofoxide layer, regardless of the stage of processing, layer exposed, orfunction of the opening.

[0009] The positions and sizes of the openings are defined byphotolithographic masks. Typically, a photosensitive film (or resist) isdeposited on the surface of a dielectric layer, and thephotolithographic mask blocks a portion of the light which wouldotherwise expose a corresponding portion of the film when the film isexposed to a light of a known intensity and frequency. Suitablephotoresist materials are those conventionally known to those ofordinary skill in the art and may comprise either positive or negativephotoresist materials. Either or both positive and/or negative resistlayers may be used. The photoresist may be applied by conventionalmethods known to those of ordinary skill in the art.

[0010] Negative resist materials may contain chemically inert polymercomponents such as rubber and/or photoreactive agents that react withlight to form cross-links, e.g. with the rubber. When placed in anorganic developer solvent, the unexposed and unpolymerized resistdissolves, leaving a polymeric pattern in the exposed regions. Thepreparation of suitable negative resist materials is within the level ofskill of one of ordinary skill in the art without undue experimentation.Specific non-limiting examples of suitable negative resist systemsinclude cresol epoxy novolac-based negative resists as well as negativeresists containing the photoreactive polymers described in Kirk-OthmerEncyclopedia of Chemical Technology, 3rd Edition, Vol. 17, entitled“Photoreactive Polymers”, pages 680-708, the relevant portions of whichare hereby incorporated by reference.

[0011] Positive resists have photoreactive components which aredestroyed in the regions exposed to light. Typically the resist isremoved in an aqueous alkaline solution, where the exposed regiondissolves away. The preparation of suitable positive resist materials iswithin the level of skill of one of ordinary skill in the art withoutundue experimentation. Specific non-limiting examples of suitablepositive resist systems include Shipley XP9402, JSR KRK-K2G and JSRKRF-L7 positive resists as well as positive resists containing thephotoreactive polymers described in Kirk-Othmer Encyclopedia of ChemicalTechnology, 3rd Edition, Vol. 17, entitled “Photoreactive Polymers”,pages 680-708, the relevant portions of which are hereby incorporated byreference.

[0012] Exemplary resist materials are also described by Bayer et al.,IBM Tech. Discl. Bull. (USA) Vol. 22, No. 5, (October 1979), pp. 1855;Tabei, U.S. Pat. No. 4,613,404; Taylor et al., J. Vac. Sci., Technol.Bull. Vol. 13, No. 6, (1995), pp. 3078-3081; Argritis et al., J. Vac.Sci., Technol. Bull., Vol. 13, No. 6, (1995), pp. 3030-3034; Itani etal., J. Vac. Sci, Technol. Bull. Vol. 13, No. 6, (1995), pp. 3026-3029;Ohfuli et al., J. Vac. Sci, Technol. Bull. Vol. 13, No. 6, (1995), pp.3022-3025; Trichkov et al., J. Vac. Sci., Technol. Bull. Vol. 13, No. 6,(1995), pp. 2986-2993; Capodieci et al., J. Vac. Sci, Technol. Bull.Vol. 13, No. 6, (1995), pp. 2963-2967; Zuniga et al., J. Vac. Sci,Technol. Bull. Vol. 13, No. 6, (1995), pp. 2957-2962; Xiao et al., J.Vac. Sci, Technol. Bull. Vol. 13, No. 6, (1995), pp. 2897-2903; Tan etal., J. Vac. Sci, Technol. Bull. Vol. 13, No. 6, (1995), pp. 2539-2544;and Mayone et al., J. Vac. Sci, Technol. Vol. 12, No. 6, pp. 1382-1382.The relevant portions of the above-identified references which describethe preparation of resist materials are hereby incorporated byreference.

[0013] In recent years, the degrees of integration of semiconductordevices have been greatly improved, and accordingly, the size reductionof various elements formed on semiconductor substrates has become one ofthe essential technical requirements. In order to meet such arequirement, it is necessary to reduce the gap between respective gates(electrodes) formed above a semiconductor substrate, and when a contacthole is formed between such gates, it is also necessary to reduce thesize of the contact hole. As the gap between gates has decreased, it hasbecome difficult to form a microscopic contact hole at an accurateposition due to limitations on the stepper alignment, etc. In recentyears, therefore, a self-aligned contact method has been used where aprotective film (base) (such as a silicon nitride(SiN_(x)) film) isformed on the surface of each gate to prevent the etching of the gatesduring the formation of a contact hole and wherein a contact hole isformed in the microscopic space between adjacent gates in aself-aligning manner.

[0014] Prior to etching, a photoresist (or film) is applied, exposed,and developed. Development of the film removes a portion of the film,thereby forming a pattern in which portions of the oxide are exposed.The exposed portions of the oxide may then be subject to selectiveetching to form a contact. Plasma-based etching processes such asreactive ion etching (RIE) are very common, however etching can also beperformed by other methods, such as using a high density chamber or aDRM chamber. In the reverse process, deposition can be performed byplasma enhanced chemical vapor deposition. Typically, the plasma isgenerated by coupling radio frequency (RF) electromagnetic energy to theplasma. The RF energy is supplied by an RF generator coupled to a powersupply. Since the plasma has a variable impedance, a matching network isemployed to match the impedance of the power supply with that of theplasma. The matching network may include one or more capacitors and oneor more inductors to achieve the match and thereby tune the RF power.Typically, the tuning may be done automatically by an automatic matchingnetwork (AMN). When tuned, most of the power output of the RF generatoris coupled to the plasma. The power. to the plasma is often referred toas forward power.

[0015] Etch characteristics are generally believed to be affected bypolymer residues which deposit during the etch. For this reason, thefluorine to carbon ratio (F/C) in the plasma is considered an importantfactor in the etch. In general, a plasma with a high F/C ratio will havea faster etch rate than a plasma with a low F/C ratio. At very low F/Cratios (i.e., high carbon content), polymer deposition may occur andetching may be reduced. The etch rate as a function of the F/C ratio istypically different for different materials. This difference is used tocreate a selective etch, by attempting to use a gas mixture which putsthe F/C ratio in the plasma at a value that leads to etching at areasonable rate for one material, and that leads to little or no etchingor polymer deposition for another. For a more thorough discussion ofoxide etching, see S. Wolf and R. N. Tauber, Silicon Processing for theVLSI ERA, Volume 1, pp 539-585 (1986), the contents of which areincorporated herein by reference. The introduction of oxygen into anetching process has been reported to allow for control of theanisotropy, by varying the fraction of O₂ in the feed. For example, seeBurton et al., J. Electrochem. Soc.: Solid-State Science and Technology,v 129, no 7, 1599 (1982), the contents of which are incorporated hereinby reference.

[0016] A number of gases have been used in known systems to etch oxidelayers, including CHF₃, CF₄, and CH₂F₂. By selecting the appropriate gasfor use in the etching process, some layers are selectively etched whileleaving other layers (etch stops) relatively unharmed. CH₂F₂ has alsobeen used and tends to provide a passivation layer on horizontalsurfaces. A mixed gas obtained by adding CO to C₄F₈ is known to be usedduring etching to form contact holes, especially during an etchingprocess which forms a contact hole between gates and through aninsulating film such as an SiO₂ film.

[0017] C₄F₈, however, is not easily decomposed in the atmosphere.Consequently, any C₄F₈ that is not dissociated during processing andwhich is subsequently released into the atmosphere contributes togreenhouse effects and thus accelerates global warming. In other words,according to “PFC Problems in Semiconductor Mass Production Plants:Current States and Countermeasures,” Climate Change 1995, theatmospheric life of C₄F₈ is 3,200 years, whereas the atmospheric life ofC₅F₈ is 0.3 years.

[0018] Previous unsuccessful attempts have been made to use C₅F₈ in aplasma processing systems. Such attempts were unsuccessful since theyfailed to provide the required selectivity. It is believed that a majorimpediment to the use of C₅F₈ was the low purity of previously availableC₅F₈ gases. Generally, available C₅F₈ gases were only 95-97% pure. As aresult, contamination created an unacceptable, non-uniform productbecause of non-uniform etch and deposition rates.

[0019] Accordingly, C₅F₈ was not previously accepted as an etchant foruniformly processing semiconductor substrates. Furthermore, the highselectivity of C₅F₈ for oxide versus silicon nitrides was unproven.C₅F₈, in the form of octofluro-cyclopentene is now available in highpurity from Nippon Zeon Co., Ltd. of 2-6-1 Marunouchi, Chiyoda-ku, Tokyo100 Japan.

SUMMARY OF THE INVENTION

[0020] It is an object of the present invention to provide an oxideetching method in which etching is effectively stopped at an etch stop.

[0021] Another object of the present invention is to provide an etchingmethod which achieves a high selectivity ratio of oxide etch to etch ofan etch stop.

[0022] These and other aspects of the present invention are madepossible by a first embodiment of an improved gas plasma processingsystem utilizing a mixture of a high purity C₅F₈ gas in combination witha carrier gas (such as Ne, Kr, Xe, He, Ar, and N₂).

[0023] These and other objects are also made possible by a secondembodiment of an improved gas plasma processing system utilizing amixture of a high purity C₅F₈ gas in combination with a carrier gas andat least one of CO and O₂. In both the first and second embodiments, themixture is introduced into a hermetic treatment chamber holding asubstrate to be processed, wherein the substrate includes an SiO₂ layerabove an SiN_(x) layer.

[0024] It is known that C₅F₈ is decomposed in the atmosphere within arelatively brief period of time in comparison to other C_(x)F_(y) gaseswhich have heretofore been employed as treatment gases, such as CF₄,C₂F₆, C₄F₈. When C₅F₈ is employed as a treatment gas, therefore, C₅F₈does not contribute to the greenhouse effect even if it is directlyreleased into the atmosphere, thereby preventing global warming. C₅F₈,furthermore, is more carbon-rich than the C₅F_(y) gases such as CF₄,C₂F₆, C₄F₈, and, therefore, it is easily capable of forming a carbonlayer on the treatment surface of a substrate. As a result, theselection ratio is improved, and a desired etching process is performedon the treatment object (e.g., silicon wafer). In addition, when usingC₅F₈ and O₂ to form a contact hole, the quantity of a polymer generatedwithin the contact hole can be controlled due to the presence of O₂within the gas mixture. As a result, the contact hole angle can becontrolled, and the occurrences of etching stoppages can also beprevented.

[0025] The above objects also are realized by a third embodiment thatutilizes a gas mixture introduced into a hermetic treatment chamber thathouses a substrate having an SiO₂ layer above an SiN_(x) layer. Theprocess etches the SiO₂ layer using a gas mixture that includes at leastC₄F₈ and CO until the SiN_(x) layer is exposed, and subsequently etchesthe SiO₂ layer using C₅F₈ and at least one of CO and O₂ after saidSiN_(x) layer has been exposed.

[0026] According to the third embodiment, a gas mixture which includesC₄F₈ is used only until the SiN_(x) layer is exposed, and therefore, therelative quantity of C₄F₈, which is not easily decomposed can bereduced. If C₄F₈ is thus used until the exposure of the SiN_(x) layerwhen a contact hole is formed on an SiO₂ layer, then the formation of acarbon layer is inhibited, as a result the bottom plane of the contacthole can be properly etched. If C₅F₈ is used after the SiN_(x) layer hasbeen exposed, a carbon layer also can easily be formed, and theselection ratio of the SiO₂ with respect to SiN_(x) can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] A more complete appreciation of the invention and many of theattendant advantages thereof will become readily apparent with referenceto the following detailed description, particularly when considered inconjunction with the accompanying drawings, in which:

[0028]FIG. 1 is a schematic illustration of one embodiment of a gasplasma processing system;

[0029]FIG. 2 is a schematic illustration showing an approximatemagnified cross-sectional view of a wafer which is treated by using thegas plasma processing system shown in FIG. 1;

[0030]FIGS. 3A and 3B are schematic illustrations showing approximateetches provided at a middle and an edge of a wafer, respectively, whenetching occurs in the gas plasma processing system shown in FIG. 1 inthe presence of C₅F₈/CO/Ar in quantities of 6/300/380 sccm;

[0031]FIGS. 4A and 4B are schematic illustrations showing approximateetches provided at a middle and an edge of a wafer, respectively, whenetching occurs in the gas plasma processing system shown in FIG. 1 inthe presence of C₅F₈/CO/Ar in quantities of 9/300/380 sccm;

[0032]FIGS. 5A and 5B are schematic illustrations showing approximateetches provided at a middle and an edge of a wafer, respectively, whenetching occurs in the gas plasma processing system shown in FIG. 1 inthe presence of C₅F₈/CO/Ar in quantities of 6/150/380 sccm;

[0033]FIGS. 6A and 6B are schematic illustrations showing approximateetches provided at a middle and an edge of a wafer, respectively, whenetching occurs in the gas plasma processing system shown in FIG. 1 inthe presence of C₅F₈/CO/Ar in quantities of 6/150/600 sccm;

[0034]FIGS. 7A and 7B are schematic illustrations showing approximateetches provided at a middle and an edge of a wafer, respectively, whenetching occurs in the gas plasma processing system shown in FIG. 1 inthe presence of C₅F₈/O₂/Ar in quantities of 6/4/380 sccm;

[0035]FIGS. 8A and 8B are schematic illustrations showing approximateetches provided at a middle and an edge of a wafer, respectively, whenetching occurs in the gas plasma processing system shown in FIG. 1 inthe presence of C₅F₈/O₂/Ar in quantities of 6/6/380 sccm;

[0036]FIGS. 9A and 9B are schematic illustrations showing approximateetches provided at a middle and an edge of a wafer, respectively, whenetching occurs in the gas plasma processing system shown in FIG. 1 inthe presence of C₅F₈/O₂/Ar in quantities of 6/4/600 sccm;

[0037]FIGS. 10A and 10B are schematic illustrations showing approximateetches provided at a middle and an edge of a wafer, respectively, whenetching occurs in the gas plasma processing system shown in FIG. 1 inthe presence of C₅F₈/O₂/Ar in quantities of 3/2/190 sccm;

[0038]FIGS. 11A and 11B are schematic illustrations showing approximateetches provided at a middle and an edge of a wafer, respectively, whenetching occurs in the gas plasma processing system shown in FIG. 1 inthe presence of C₅F₈/O₂/Ar in quantities of 12/8/760 sccm;

[0039]FIGS. 12A and 12B are schematic illustrations showing approximateetches provided at a middle and an edge of a wafer, respectively, whenetching occurs in the gas plasma processing system shown in FIG. 1 inthe presence of C₅F₈/O₂/Ar in quantities of 6/4/380 sccm at a chamberpressure of 30 mTorr;

[0040]FIGS. 13A and 13B are schematic illustrations of an approximatelyideal self-aligning contact on a partially processed semiconductor waferhaving an anti-reflective coating before and after processing,respectively, according to the present invention;

[0041]FIGS. 14A and 14B are schematic illustrations of an approximatelyideal self-aligning contact on a partially processed semiconductor waferwithout an anti-reflective coating before and after processing,respectively, according to the present invention;

[0042]FIGS. 15A and 15B are schematic illustrations of an approximatelyideal contact on a partially processed semiconductor wafer before andafter processing, respectively, according to the present invention;

[0043]FIGS. 16A and 16B are schematic illustrations of an approximatelyideal dual damascene structure on a partially processed semiconductorwafer before and after processing, respectively, according to thepresent invention;

[0044] FIGS. 17A-17D are tables of results showing the effects of C₅F₈and additional gases on the etching of layers of oxide, SiN, oxide afterashing, and SiN after ashing, respectively;

[0045]FIGS. 18A and 18B are graphs showing deposition rates for oxideand nitride, respectively, on a blank wafer as a function of flow rateof C₅F₈ when the flow rates of CO and O₂ were 100 sccm and 0 sccm,respectively; and

[0046] FIGS. 19A-19E are graphs of approximated linear fits ofselectivity, etching rate of an oxide layer, uniformity of a resultingoxide layer, uniformity of a nitride layer, and etching of a nitridelayer, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] Turning now to the drawings in which like reference numeralsdesignate identical or corresponding parts throughout the several views,FIG. 1 is a schematic illustration of one embodiment of the presentinvention. In this embodiment, the treatment chamber (102) of theetching device (100) is formed within the hermetic and electroconductivetreatment container (104). Magnets (106), which are capable of forming arotating magnetic field in a plasma region formed within the treatmentchamber (102), surround the treatment container (104). The upperelectrode (108), which constitutes the ceiling of the treatment chamber(102), and the electroconductive lower electrode (110), opposite theupper electrode (108) and which constitutes a susceptor, are formedwithin the treatment chamber (102).

[0048] The electrostatic chuck (112), to which the high-voltage DC powersource (114) is connected, is configured above the lower electrode(110). A treatment object, such as the semiconductor wafer (hereafterreferred to simply as the “wafer”) (W), can be fixed via suction to themounting plane above this electrostatic chuck (112). A lift mechanism(not shown in the figure) is connected to the lower electrode (110) viathe lift axle (116), and the lower electrode (110) can freely ascend anddescend. The focus ring (118) is positioned to cover the profile planeof the lower electrode (110). The baffle panel (120), on which multiplethrough holes have been formed, is attached to the profile plane of thefocus ring (118). The high-frequency power source (124), which iscapable of outputting a certain high-frequency electrical power forforming a plasma, is connected to the lower electrode (110).

[0049] Multiple through holes (108 a), which connect the interior of thetreatment chamber (102) with the gas feeding source (126) are formed onthe upper electrode (108) creating a “shower head” shape. Based on sucha structure, the treatment gas employed in the present embodiment (e.g.,mixed gas including C₅F₈, O₂, and a carrier gas) is homogeneouslyejected in the wafer (W) direction within the treatment chamber (102)from the gas feeding source (126) via the through holes (108 a). Thetreatment gas, however, is not limited to the aforementioned mixed gasincluding C₅F₈, O₂, and the carrier gas, and depending on the types ofwafers (W) and treatment conditions, it is possible to use (1) C₅F₈alone or (2) a gas mixture including C₅F₈ and at least one of O₂ and CO.In the present invention, high purity C₅F₈, in the form ofoctafluro-cyclopentene from Nippon Zeon Co., Ltd. is used. It is alsopossible to add carrier gases which do not react with the substratebeing processed; the carrier gases including Ne, Kr, Xe, He, Ar, N₂,etc. The functions and effects of varying gases is explained usingseveral examples below.

[0050] The atmosphere within the treatment chamber (102) is dischargedfrom the gas exhaustion tube (128) via the baffle panel (120). Theatmospheric pressure within the treatment chamber (102) is selecteddepending on the rate at which the aforementioned treatment gas is fedinto the treatment chamber (102) and the atmosphere discharge levelwithin the treatment chamber (102).

[0051] A process for etching a contact hole in the wafer (W) by usingthe etching device (100) according to the present invention is explainedwith reference to FIG. 2. First, the gates (202) are formed above the Si(silicon) substrate (200) (e.g., the wafer (W)), and the insulatinglayer (204) is formed to cover the gates (202). The Si₃N₄ film (siliconnitride) (206) covers the surface of the insulating layer (204). ThisSi₃N₄ film (206) prevents the etching of the gates (202) during theformation of the contact hole (210), and forms the contact hole (210)between the gates (202) in a self-aligning fashion. A silicon oxide filmacts as an insulating layer is formed above the Si₃N₄ film (206). Theinsulating layer (204) and SiO₂ film (208) may also be constituted byBPSG, PSG, TEOS (tetraethoxy-o-silane), Th-OX (thermal oxide), SOG(spun-on glass), etc.

[0052] An oxide is formed over the wafer to isolate one layer from thenext layer. A photoresist is then applied (e.g., using spin coating)after optionally having applied an anti-reflective coating. Non-limitingexamples of suitable techniques for applying a photoresist will includespraying, roller coating and spin coating. Examples of preferred resistmaterials include “Deep UV” for use with anti-reflective coatings and“I-line” for use without an anti-reflective coating. The thickness ofthe photoresist material may vary depending upon the application.However, according to the invention, a thinner photoresist layer can beused in light of the high selectively of the CF₈. The photoresist isthen developed to remove a corresponding portion of the photoresist,leaving behind the pattern of holes to be etched. After patterning anddeveloping, hard baking of the resist may be conducted by conventionalmethods known to those of ordinary skill in the art. Hard baking istypically performed to prevent photoresist lifting from the underlyingdielectric layer during wet etching, for example, by baking at about120-130° C. It may be required to re-expose portions of the oxide layerafter baking using a breakthrough or “descum” procedure which typicallyutilizes either Ar and O₂ or CHF₃ and SO₂.

[0053] To form a contact hole, the wafer (W) first is mounted above thelower electrode (110), and the treatment gas of the present applicationembodiment (e.g., a mixed gas including C₅F₈, O₂, and Ar) is introducedinto the treatment chamber (102) while vacuum suction is applied tomaintain a certain reduced atmospheric pressure (e.g., 40 (mTorr))within the treatment chamber (102). Moreover, the magnets (106) arerotated to form a rotating magnetic field in the plasma region withinthe treatment chamber (102). Next, after certain process conditions havebeen established, a high-frequency electrical power with a certainmagnitude (e.g., 1,500 (W)) is applied on the lower electrode (110) fromthe high-frequency power source (124) at a certain frequency (e.g.,13.56 (MHZ)), and a glow discharge is induced between it and the upperelectrode (108). As a result, the treatment gas of the presentapplication embodiment, which has been fed into the treatment chamber(102), is dissociated, and a high-density plasma is excited.

[0054] Since the treatment gas of the present application embodiment isa mixed gas including C₅F₈, O₂, and a carrier gas, it is capable ofcreating a carbon-rich atmosphere within the treatment chamber (102)without adding CO, and a carbon layer which serves as a protective layercan be assuredly formed on the inner wall surface of the contact hole.(In an alternate embodiment described below, CO is added.) As a result,the arrival of fluorine radicals (i.e., etching ions) at the siliconnitride film (206) is inhibited, and since the SiN_(x) film (206) can beprotected, the selection ratio between the SiN_(x) film (206) and theSiO₂ film (208) can be improved. As FIG. 2 indicates, the contact hole(210) can be formed in the narrow space between the gates (202) to havean objective homogeneous shape. The photoresist (212), which serves as amask for the contact hole (210), is formed above the SiO₂ film (208).

[0055] Based on the foregoing structure of the present applicationembodiment, an etching process is performed by using a gas mixture whichincludes C₅F₈, which can be decomposed within a relatively brief periodin the atmosphere, during the formation of the contact hole (210).Accordingly, any C₅F₈ not decomposed during the treatment, and even anyC₅F₈ accidentally released directly into the atmosphere, contributesless to greenhouse effects than previous C_(x)F_(y) gases.

[0056] In the following application examples of the use of C₅F₈, theetching device (100) is used to form a contact hole (210) between thegates (202) which have been formed on the wafer (W). By changing variousconditions, such as the treatment gas composition, the gas flow rate,etc., various differences in a resulting etch are obtained.

[0057] As shown in FIGS. 3A and 3B, a gas mixture of C₅F₈ and CO is usedto etch a substrate. The atmospheric pressure within the treatmentchamber (102) was selected as 40 (rnTorr), and the temperatures of theupper electrode (108) and the inner wall surface of the treatmentchamber (102) were maintained at 60 (° C.), whereas the temperature ofthe lower electrode (110) was maintained at 40 (° C.). A high-frequencyelectrical power of 1,500 (W) was applied on the lower electrode (110)at a frequency of 13.56 (MHZ). A chamber with a diameter of 200 (mm) wasused with a wafer (W). A treatment was performed by using an etchingdevice (100) equipped with a treatment chamber (102) with a contentvolume of 38 (L).

[0058]FIGS. 3A through 6B show the homogeneities within the treatmentsurface plane of the wafer (W) (hereafter referred to simply as the“intraplane homogeneity”) at variable treatment gas flow rates. Theapproximate corresponding shapes of the wafers (W) are also shown.Further, the selection ratios of SiO₂ with respect to SiN_(x) werecalculated for each of the rates. The intraplane homogeneity isinversely proportional to the deviation between the average etchingrates of the middle and edge of the wafer (W) versus the average etchingrate (%). As the absolute value of the deviation increases, thehomogeneity decreases, whereas as it decreases, the homogeneityincreases. The selection ratio of SiO₂ with respect to SiN_(x) is theaverage of the (SiO₂ etching rate)/(SiN_(x) etching rate) values of themiddle and edge of the wafer (W) in a state where the SiN_(x) film (206)is exposed during etching of the SiO₂ film (208).

[0059] In a first example corresponding to FIGS. 3A and 3B, the flowrates of the treatment gas combinations were established as:C₅F₈/CO/Ar=6/300/380 (sccm), and an etch was performed under conditionsotherwise identical to the aforementioned ones. As a result, the etchingrate of SiO₂ at the middle of the wafer (W) was 3,200 (Å/min.), and thecorresponding rate at the edge was also 3,200 (Å/min.). The intraplanehomogeneity of the wafer (W) was ±0(%), and the selection ratio of SiO₂with respect to SiN_(x) was 9.6.

[0060] In a second example corresponding to FIGS. 4A and 4B, the flowrate of C₅F₈ was changed from 6 sccm to 9 sccm to provide flow rates of:C₅F₈/CO/Ar=9/300/380 (sccm), and an etch was performed under the otherpreviously described conditions. As a result, the etching rate of SiO₂at the middle of the wafer (W) was 3,600 (Å/min.), and the correspondingrate at the edge was 4,100 (Å/min.). The intraplane homogeneity of thewafer (W) was ±6.5(%), and the selection ratio of SiO₂ with respect toSiN_(x) was 23.1.

[0061] In a third example corresponding to FIGS. 5A and 5B, the flowrates of the treatment gas combinations were established as:C₅F₈/CO/Ar=6/150/380 (sccm), and an etch was performed under conditionsotherwise identical to the aforementioned ones. As a result, the etchingrate of SiO₂ at the middle of the wafer (W) was 3,700 (Å/min.), and thecorresponding rate at the edge was 4,100 (Å/min.). The intraplanehomogeneity of the wafer (W) was ±5.1(%), and the selection ratio ofSiO₂ with respect to SiN_(x) was 11.4.

[0062] In a fourth example corresponding to FIGS. 6A and 6B, the flowrates of the treatment gas combinations were established asC₅F₈/CO/Ar=6/150/600 (sccm), and an etch was performed under conditionsotherwise identical to the aforementioned ones. As a result, the etchingrate of SiO₂ at the middle of the wafer (W) was 3,300 (Å/min.), and thecorresponding rate at the edge was 3,500 (Å/min.). The intraplanehomogeneity of the wafer (W) was ±2.9(%), and the selection ratio ofSiO₂ with respect to SiN_(x) was 12.0.

[0063] Thus, when SiO₂ was selectively etched in relation to SiN_(x) byusing a mixed gas of C₅F₈, CO, and Ar and where the flow rates of therespective components of the mixed gases were varied according to theconditions of the aforementioned first through fourth examples, theetching rates ranged from 3,200 (Å/min.) to 4,100 (Å/min.), whereas theintraplane homogeneities ranged from 0(%) to ±6.5(%), and the selectionratios of SiO₂ with respect to SiN_(x) ranged from 9.6 to 23.1. Thus,contact holes with homogeneous shapes can be formed by varying the flowrates of the respective constituent gases of the treatment gases, asdescribed in the above examples.

[0064] In an alternate embodiment of the present invention, a mixture ofC₅F₈ and O₂ is described. In the following fifth through tenth examples,which respectively correspond to FIGS. 7A through 12B, the designatedconditions other than the treatment gases and the flow rates of thetreatment gases were identical to those in the aforementioned firstthrough fourth examples, in which a mixture of C₅F₈ and CO was used. Inthe tenth example corresponding only to FIG. 12, the atmosphericpressure within the treatment chamber (102) was changed from 40 mTorr to30 mTorr. In the fifth through tenth examples, the intraplanehomogeneities of the wafers (W) and the selection ratios Of SiO₂ withrespect to SiN_(x) were respectively computed according to proceduresidentical to those in the aforementioned first through fourthapplication examples, and the approximate etched shapes of the wafers(W) are shown.

[0065] In a fifth example corresponding to FIGS. 7A and 7B, the flowrates of the treatment gas combinations were established asC₅F₈O₂/Ar=6/4/380 (sccm), and an etch was performed under conditionsotherwise identical to those in the aforementioned first through fourthexamples. As a result, the etching rate of SiO₂ at the middle of thewafer (W) was 4,500 (Å/min.), and the corresponding rate at the edge was5,200 (Å/min.). The intraplane homogeneity of the wafer (W) was ±7.2(%),and the selection ratio of SiO₂ with respect to SiN_(x) was 16.8.

[0066] In a sixth example corresponding to FIGS. 8A and 8B, the flowrates of the treatment gas combinations were established asC₅F⁸O₂/Ar=6/6/380 (sccm), and an etch was performed under conditionsotherwise identical to those in the aforementioned fifth example. Inother words, only the flow rate of O₂ was changed from the 4 (sccm) inthe aforementioned fifth example to 6 (sccm) in the present example. Asa result, the etching rate of SiO₂ at the middle of the wafer (W) was4,700 (Å/min.), and the corresponding rate at the edge was also 4,700(Å/min.). The intraplane homogeneity of the wafer (W) was ±0(%), and theselection ratio of SiO₂ with respect to SiN_(x) was 13.3.

[0067] In a seventh example corresponding to FIGS. 9A and 9B, the flowrates of the treatment gas combinations were established asC₅F₈/O₂/Ar=6/4/600 (sccm), and an etch was performed under conditionsotherwise identical to those in the aforementioned fifth example.

[0068] In other words, only the flow rate of Ar was changed from the 380(sccm) in the aforementioned fifth example to 600 (sccm) in the presentexample. As a result, the etching rate of SiO₂ at the middle of thewafer (W) was 3,900 (Å/min.), and the corresponding rate at the edge was4,200 (Å/min.). The intraplane homogeneity of the wafer (W) was ±3.7(%),and the selection ratio of SiO₂ with respect to SiN_(x) was 14.7.

[0069] In an eighth example corresponding to FIGS. 10A and 10B, the flowrates of the treatment gas combinations were established asC₅F₈/O₂/Ar=3/2/190 (sccm), and an etch was performed under conditionsotherwise identical to those in the aforementioned fifth example. Inother words, the respective flow rates of C₅F₈, O₂, and Ar were reducedin half in the present example in comparison with the flow rates ofC₅F₈, O₂, and Ar in the aforementioned fifth example. As a result, theetching rate of SiO₂ at the middle of the wafer (W) was 4,000 (Å/min.),and the corresponding rate at the edge was 3,900 (Å/min.). Theintraplane homogeneity of the wafer (W) was ±1.3(%), and the selectionratio of SiO₂ with respect to SiN_(x) was 10.8.

[0070] In a ninth example corresponding to FIGS. 11A and 11B, the flowrates of the treatment gas combinations were established asC₅F₈/O₂/Ar=12/8/760 (sccm), and an etch was performed under conditionsotherwise identical to those in the aforementioned fifth example. Inother words, the respective flow rates of C₅F₈, O₂, and Ar were doubledin the present example in comparison with the flow rates of C₅F₈, O₂,and Ar in the aforementioned fifth example. As a result, the etchingrate of SiO₂ at the middle of the wafer (W) was 4,200 (Å/min.), and thecorresponding rate at the edge was also 4,200 (Å/min.). The intraplanehomogeneity of the wafer (W) was ±0(%), and the selection ratio of SiO₂with respect to SiN_(x) was 12.3.

[0071] In a tenth example corresponding to FIGS. 12A and 12B, theatmospheric pressure within the treatment chamber (102) was lowered to30 (mTorr), and an etch was performed under conditions otherwiseidentical to those in the aforementioned fifth example. In other words,the flow rate of the treatment gas was designated at C₅F₈/O₂/Ar=6/4/380(sccm), as in the aforementioned fifth example, and only the atmosphericpressure within the treatment chamber (102) was changed from the 40(mTorr) in the aforementioned fifth example to 30 (mTorr). As a result,the etching rate of SiO₂ at the middle of the wafer (W) was 4,400(Å/min.), and the corresponding rate at the edge was 4,500 (Å/min.). Theintraplane homogeneity of the wafer (W) was ±1.1(%), and the selectionratio of SiO₂ with respect to SiN_(x) was 13.1.

[0072] Thus, in cases where the flow rates of the respective componentsof a mixed gas of C₅F₈, O₂, and Ar were varied according to theconditions of the aforementioned fifth through ninth examples, and whenthe atmospheric pressure within the treatment chamber (102) was variedaccording to the conditions of the aforementioned tenth example duringthe selective etching of SiO₂ in relation to SiN_(x) by using said mixedgas, the etching rates ranged from 3,900 (Å/min.) to 5,200 (Å/min.),whereas the intraplane homogeneities ranged from 0(%) to ±7.2(%), andthe selection ratios of SiO₂ with respect to SiN_(x) ranged from 10.8 to16.8. Thus, contact holes with homogeneous shapes can be formed byvarying the flow rates of the respective constituent gases as in thefifth through tenth examples above.

[0073] In another alternate embodiment, a contact hole is formed on thewafer (W) more effectively and efficiently by etching an SiO₂ film byusing a mixed gas of CO and C₄F₈ (the etching rate of which versus theSiO₂ film is higher than that of C₅F₈) until the SiN_(x) film formedabove the wafer (W) is exposed, then subsequently by etching the SiO₂film by switching to a mixed gas of O₂, Ar, and C₅F₈, which yields anSiO₂ film selection ratio versus the SiN_(x) film higher than that ofC₄F₈, or to a mixed gas of C₅F₈, CO, and Ar after said SiN_(x) film hasbeen exposed.

[0074] In an alternate embodiment, a mixture of gases is used to etch aportion of a partially exposed wafer, e.g., to expose a self-aligningcontact such as shown in FIG. 13A. An uppermost photoresist layer hasbeen applied on top of a 900 Å anti-reflective coating (ARC), whichhelps to ensure good patterning during the photolithography step. Thelayer shown has been partially stripped during development to expose a0.4 μm hole above a portion of the wafer to be processed. The exposedportion to be etched includes a 7500 Å BPSG dielectric layer which isformed on top of a 400 Å SiN layer that acts as an etch stop. As wouldbe evident to one of ordinary skill in the art, other dielectric layersand other etch stops are also possible. Generally, the etch stopsdescribed herein will be referred to as silicon nitride layers, but theinvention is applicable to any Si_(x)N_(y) layer. A preferred dielectriclayer includes SiO₂ or SiO₂ doped with boron (B) and/or phosphorous (P).In another embodiment, the dielectric material is a TEOS layer dopedwith boron (B) and/or phosphorous (P) to form BTEOS, PTEOS, or BPTEOS.When the dielectric material is BPTEOS, the dielectric layer may furthercomprise a capping layer prepared from TEOS, which can act to stabilizethe BPTEOS layer during processing and/or prevent etching and/ormigration of dopants from the BPTEOS layer into a subsequently depositedlayer. When present, a capping layer may be etched in a first stage,under optimum etching conditions which are not necessarily the optimumetching conditions for the underlying BPTEOS layer. After the cappinglayer has been etched, the underlying BPTEOS layer may be etchedaccording to the method described herein.

[0075] The etching process according to the present invention is moreproperly characterized as a “depth etching” step rather than a chemicalprocessing step because C₅F₈ does not always react with oxides orsilicon nitride to etch these layers. Instead, the amount of C₅F₈ andother gases adjusts a balance between a sputter/etch rate and adeposition rate of a protective polymer that is produced in the presenceof carbon and flourine. An increase in the amount of O₂ decreases thedeposition rate of the polymer and reduces the selectivity of the C₅F₈.However, if too little O₂ is provided, the deposition rate of thepolymer is increased so substantially that the deposition preventsetching (almost) completely. The present invention, therefore, seeks tomaintain a high selectivity by limiting the amount of O₂, but withoutreducing the O₂ concentration to the point that an overly aggressivepolymer deposition process is created.

[0076] As seen in FIG. 13B, when the gas mixture is properly balanced,the etch process stops at the etch stop without damaging the oxide layerunderneath. This high selectivity is especially important when thissilicon nitride layer is a plasma enhanced nitride layer which is softerthan other silicon nitride layers grown in a furnace with silane andnitrogen. In order to protect the shoulders of the 400 Å SiN layer frombeing etched away, the present invention utilizes a mixture of C₅F₈ incombination with a carrier gas and at least one of CO and O₂ It ispreferred that the C₅F₈ gas be of very high purity—in the range ofgreater than 97% pure and preferably at least 99% pure. The gases are tobe mixed such that the flow rates (measured in standard cubiccentimeters per minute (sccm)) are in the ranges of: C₅F₈:7-15; CO:0-100; carrier gas: 500; and O₂: 0-6. It is believed that the additionof CO provides an additional amount of free carbons that increase thepolymer deposition rate. Too little CO reduces the selectivity of theC₅F₈ and increases the etch rate. However, when too much CO is present,too much polymer is produced. To restore the balance between the etchrates of oxide and silicon nitride, O₂ can be added to the process toreduce the rate of production of the polymer, thereby increasing theetch rate of the oxide (as can be seen by comparing runs #7 and #8 inFIG. 17A). Using the combination of C₅F₈, CO, O₂, and a carrier gascreates a quasi-equilibrium in which sputtering and etching occursimultaneously. As seen for run #6 in FIGS. 6A and 6B, using rates of 9,50, 500, and 3, respectively, for C₅F₈, CO, Ar and O₂, provides areasonably fast etch rate for oxide (3235 Å/min at maximum) and arelatively low etch rate for SiN (29 Å/min at max and only 13 Å/min onaverage). On average, the oxide is etched twenty times faster than theSiN. This provides the desired selectivity in which the oxide is etchedbut the SiN layer is left, relatively, less etched or unetched.

[0077] After the etch step, a polymer removal step may be required toremove the polymer deposited in the presence of the C₅F₈ gas mixture.The polymer is believed to be a strong carbon-based polymer and can beremoved with O₂. As can further be seen in FIG. 13B, the shoulder orcorner of the silicon nitride layer above the poly layer continues toprotect the poly layer even though the oxide in the middle of theopening is etched to below the level of the top of the silicon nitride.

[0078] In FIG. 14A, another circuit structure is used to etch a 7500 ÅBPSG layer, as in FIG. 13A, but without the help of an anti-reflectivecoating layer underneath. As shown in FIG. 14B, the etch process againis stopped at the 400 Å SiN layer. The process, however, is not limitedto the etching of BPSG. A thick oxide alone can be etched to provide acontact hole above a bare silicon substrate. FIGS. 15A and 15B show suchan oxide layer before and after etch. FIGS. 16A and 16B show twodifferent size holes being etched with SiN acting as a barrier toetching of the 1.0 urn TEOS layer. The sidewalls are not necessarilystraight and tend to be shaped similarly to the photoresist above theopening.

[0079]FIG. 18A and 18B show deposition/etch rates for various flow ratesof C₅F₈. The curves however are affected by the fact that the rates weredetermined for a blank wafer rather than a patterned wafer. The blankwafer was 100% exposed to the gas mixture, whereas a patterned wafer isonly about 1% exposed. Therefore, the curves of FIGS. 18A and 18Bprobably reflect processes where the insufficient amount of gas limitsor slows the etching. It is believed that the deposition rates for oxideand silicon nitride would be reduced by approximately at least 200 Å/meach for patterned wafers. Thus, silicon nitride would undergo an etchprocess for flow rates of less than 7 sccm.

[0080] FIGS. 19A-19E show approximated linear fits of selectivity,etching rate of an oxide layer, uniformity of a resulting oxide layer,uniformity of a nitride layer, and etching of a nitride layer,respectively.

[0081] Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. In an etching method wherein a treatment gas is introduced into ahermetic treatment chamber that houses a treatment object including anSiO₂ layer above an SiN_(x) layer, the improvement comprising: using assaid treatment gas a gas including at least C₅F₈.
 2. In an etchingmethod wherein a treatment gas is introduced into a hermetic treatmentchamber that houses a treatment object including an SiO₂ layer above anSiN_(x) layer, the improvement comprising: using as said treatment gas agas including at least C₅F₈ and CO.
 3. In an etching method wherein atreatment gas is introduced into a hermetic treatment chamber thathouses a treatment object including an SiO₂ layer above an SiN_(x)layer, the improvement comprising: using as said treatment gas a gasincluding at least C₅F₈ and O₂.
 4. An etching method comprising thesteps of: etching an SiO₂ layer of a treatment object, including theSiO₂ layer above an SiN_(x) layer, in a hermetic treatment chamber usinga gas which includes at least C₄F₈ and CO until said SiN_(x) layer isexposed; and etching said SiO₂ layer, after said SiN_(x) layer has beenexposed, using one of (1) C₅F₈ and (2) C₅F₈ and at least one of CO andO₂.
 5. The etching method as claimed in claim 1, further comprising thestep of adding a carrier gas to said treatment gas.
 6. The etchingmethod as claimed in claim 2, further comprising the step of adding acarrier gas to said treatment gas.
 7. The etching method as claimed inclaim 3, further comprising the step of adding a carrier gas to saidtreatment gas.
 8. The etching method as claimed in claim 4, furthercomprising the step of adding a carrier gas to said treatment gas.
 9. Amethod for etching a semiconductor substrate, comprising the steps of:etching a semiconductor substrate in a mixture of C₅F₈, a carrier gas,O₂ and CO in quantities in a range, given in standard cubic centimetersper minute (sccm), of: 5-15 sccm, 500 sccm, 0-100 sccm, and 0-6 sccm,respectively.
 10. The method as claimed in claim 9, wherein the step ofetching comprises applying the gas mixture to etch the semiconductorsubstrate in a plasma processing system.
 11. The method as claimed inclaim 9, wherein the carrier gas is Ar.
 12. The method as claimed inclaim 9, wherein the step of etching comprises mixing C₅F₈, a carriergas, O₂ and CO in quantities in a range of: 9-11 sccm, 500 sccm, 50-100sccm, and 3-6 sccm, respectively.
 13. The method as claimed in claim 12,wherein the carrier gas is Ar.
 14. The method as claimed in claim 9,wherein the step of etching comprises mixing C₅F₈, Ar, O₂ and CO inquantities substantially of: 9 sccm, 500 sccm, 50 sccm, and 3 sccm,respectively.
 15. The method as claimed in claim 14, wherein the carriergas is Ar.
 16. A method for etching a semiconductor substrate,comprising the steps of: mixing C₅F₈, Ar, O₂ and CO; and etching anoxide layer and a silicon nitride layer at a selectivity ratio of atleast 3:1.
 17. The method as claimed in claim 16, wherein the step ofetching comprises etching an oxide layer and a silicon nitride layer ata selectivity ratio of at least 10:1.
 18. The method as claimed in claim16, wherein the step of etching comprises etching an oxide layer and asilicon nitride layer at a selectivity ratio of at least 20:1.
 19. Anetchant, comprising: C₅F₈, a carrier gas, O₂ and CO in quantities in arange, given in standard cubic centimeters (scc), of: 5-15 scc, 500 scc,0-100 scc, and 0-6 scc, respectively.
 20. The etchant as claimed inclaim 19, wherein the carrier gas is Ar.
 21. The etchant as claimed inclaim 19, wherein the quantities of C₅F₈, the carrier gas, O₂ and CO arein a range of 9-11 scc, 500 scc, 50-100 scc, and 3-6 scc, respectively.22. The etchant as claimed in claim 21, wherein the carrier gas is Ar.23. The etchant as claimed in claim 19, wherein the quantities of C₅F₈,the carrier gas, O₂ and CO are substantially: 9 scc, 500 scc, 50 scc,and 3 scc, respectively.
 24. The etchant as claimed in claim 23, whereinthe carrier gas is Ar.